Abdellatif Goumri

Experimental and theoretical studies of the reaction of atomic hydrogen with silane

Article on experimental and theoretical studies of the reaction of atomic hydrogen with silane.

The reaction of OH with acetone and acetone-d6 from 298 to 832 K: Rate coefficients and mechanism

The pulsed laser photolysis/pulsed laser-induced fluorescence technique has been applied to obtain rate coefficients for OH+CH3C(O)CH3 and CD3C(O)CD3 of kH(298–832 K)=(3.99±0.40)×10−24T4.00 exp(453±44)/T and kD(298–710 K)=(1.94±0.31)×10−21T3.17 exp(−529±68)/T cm3 molecule−1 s−1, respectively. Three pathways were characterized via the CBS–QB3 ab initio method to obtain complete basis set limits for coupled-cluster theory. Addition to form CH3C(O)(OH)CH3, followed by dissociation to CH3+CH3C(O)OH, is negligibly slow. Variational transition state theory reveals that the dominant products are CH3C(O)CH2+H2O formed by direct abstraction at higher temperatures and via a hydrogen-bonded complex below about 450 K. Inclusion of tunneling gives good accord with the observed kinetic isotope effect down to about 250 K.

Computational studies of the potential energy surface for O(3P)+H2S: Characterization of transition states and the enthalpy of formation of HSO and HOS

Structures and vibrational frequencies for minima and 11 transition states on the O(3P)+H2S potential energy surface have been characterized at the MP2=FULL/6‐31G(d) level. GAUSSIAN‐2 theory was employed to calculate ΔHf,298 for HSO and HOS of −19.9 and −5.5 kJ mol−1, respectively. The kinetics of HSO=HOS isomerization are analyzed by Rice–Ramsperger–Kassel–Marcus theory. Transition state theory analysis for O+H2S suggests OH+HS is the dominant product channel, with a rate constant given by 1.24×10−16 (T/K)1.746 exp(−1457 K/T) cm3 molecule−1 s−1. Kinetic isotope effects and the branching ratio for H+HSO production are also analyzed. The other possible products H2+SO and H2O+S do not appear to be formed in single elementary steps, but low‐barrier pathways to these species via secondary reactions are identified. No bound adducts of O+H2S were found, but results for weakly bound triplet HOSH are presented. The likely kinetics for the reactions OH+SH→S(3P)+H2O, OH+SH→cis and trans 3HOSH, cis 3HOSH→HOS+H, and ...

Theoretical studies of the RSOO, ROSO, RSO2 and HOOS (R=H, CH3) radicals

Abstract The geometries of the radicals HSOO, HOSO, HSO 2 and HOOS have been optimized at the MP2=FULL/6-31G * level, and energies obtained with Gaussian-2 theory. Internal rotations and vibrational frequencies are analyzed. The results yield values of Δ H f,298 for the four doublet radicals of 111.5, −241.4, −141.4 and 58.9 kJ mol −1 , respectively. Implications for reactions of interest in combustion and atmospheric chemistry are discussed. The results are employed to derive Δ H f,0 for CH 3 SOO, CH 3 OSO and CH 3 SO 2 of 91.8, −222.6 and −199.4 kJ mol −1 , respectively. The calculated CH 3 SOO bond strength is in excellent accord with a recent measurement.

Computational studies of the potential energy surface for O(1D)+H2S: Characterization of pathways involving H2SO, HOSH, and H2OS

Structures and vibrational frequencies for minima and transition states on the O(1D)+H2S potential energy surface have been characterized at the unrestricted second‐order Mo/ller–Plesset (UMP2)=full/6‐31G(d) level. The results for the thioperoxide HOSH agree with experimental IR spectra. Gaussian‐2 theory was employed to calculate ΔHf,298 for HOSH of −119.3 kJ mol−1, −47.1 kJ mol−1 for the sulfoxide H2SO, and 47.0 kJ mol−1 for the thiooxonium ylide H2OS. We also derived ΔHf,0 for HOS and HSO of −2.7 and −17.0 kJ mol−1, respectively. Comparisons with ΔHf for known asymptotes on the potential energy surface gave good agreement, except in the case of HSO. Rice–Ramsperger–Kassel–Marcus (RRKM) analysis suggests that in most environments, except at low pressures and temperatures, H2OS will be short lived, and rate constants for isomerization of the three bound adducts under thermally equilibrated conditions are derived. The potential energy surface is discussed in the context of single‐collision experiments, an...

Rate Constant for the Reaction C2H5 + HBr → C2H6 + Br

RRKM theory has been employed to analyze the kinetics of the title reaction, in particular, the once-controversial negative activation energy. Stationary points along the reaction coordinate were characterized with coupled cluster theory combined with basis set extrapolation to the complete basis set limit. A shallow minimum, bound by 9.7 kJ mol(-1) relative to C(2)H(5) + HBr, was located, with a very small energy barrier to dissociation to Br + C(2)H(6). The transition state is tight compared to the adduct. The influence of vibrational anharmonicity on the kinetics and thermochemistry of the title reaction were explored quantitatively. With adjustment of the adduct binding energy by ∼4 kJ mol(-1), the computed rate constants may be brought into agreement with most experimental data in the literature, including new room-temperature results described here. There are indications that at temperatures above those studied experimentally, the activation energy may switch from negative to positive.

An investigation of the gas-phase reaction of atomic bromine with disilane: implications for the Si2H5H bond strength

Article on an investigation of the gas-phase reaction of atomic bromine with disilane and implications for the Si2H5-H bond strength.